1. Field of the Invention
The present invention relates to a method for evaluating the effective flow resistance of a patient breathing circuit during mechanical breathing assistance and to a device for carrying out the method.
2. Description of the Prior Art
Providing mechanical breathing assistance to a patient is a well known medical procedure and is most often used in surgical and critical care situations. Typically, a breathing tube, such as an endotracheal or a tracheotomy tube, is inserted into the patient""s trachea so that the distal end of the tube is disposed in the patient""s airways and the proximal end is accessible external the patient. The proximal end connects with a gas tubing system, typically by means of a Y-piece connector, to form a breathing circuit which in turn connects with a breathing assist device. The breathing assist device, such as a ventilator, respirator or anaesthetic delivery system, is adapted to control the flow of gas through the breathing circuit to and from the patient""s airways and thereby regulate the patient""s breathing cycle.
Over-pressurization of the breathing gas provided to the patient through the breathing tube can cause barotrauma and therefore the gas pressure within the breathing circuit is usually monitored and used to control the assist device. Pressure sensors are typically provided within the assist device itself or at the Y-piece to monitor the gas pressures at the proximal end of the breathing tube. Because the breathing tube has a relatively narrow bore compared with the rest of the breathing circuit, the breathing tube presents a large resistance to gas flow. This leads to inaccuracies between the pressures registered by the sensors and those which exist within the lungs so that barotrauma may still occur due to a delivery pressure of the breathing gas that is too high. In other circumstances the delivery pressure of the breathing gas may be adjusted to a level that is too low so that efficient opening of the lungs no longer occurs.
In order to reduce the effects of the flow resistance of the breathing circuit (principally the delivery tube) on the safe and effective operation of the assist device it is known to adapt the device to compensate the sensed delivery pressure for the breathing tube resistance and to use this compensated pressure value to control the delivery of breathing gas. In known devices this is done by a user entering information relating to the breathing tube""s resistance before the breathing assistance is started. This information may be in the form of the tube""s length and internal diameter, from which a theoretical resistance can be calculated, or can be an actual calculated or measured resistance value determined before use.
A problem with this known approach is a possibility that the user may enter the information incorrectly. Another problem is that the resistance provided in the known manner may not be the true resistance of the tube since this may vary throughout the operation of the device or as a result of the initial placement of the tube within the patient""s trachea.
An object of the present invention to provide a method and an arrangement for carrying out the method, which make it possible to alleviate at least one of the problems associated with known breathing assist devices.
This object is achieved in accordance with the present invention in a method for evaluating an effective flow resistance of a breathing circuit during mechanical breathing assistance wherein a pressure is measured in the breathing circuit and the breathing circuit is subsequently occluded, and a pressure drop as a result of the occlusion is measured. The resistance of the breathing circuit is then determined by establishing a relationship between the measured pressures before and after the occlusion.
The determined pressure drop at the onset of an occlusion may be assumed to effectively result from the resistance of a patient breathing tube which is present within the breathing circuit since the diameter of the tube is much smaller than the diameter of any other tubing component of the circuit. An indication of the tube resistance thus can be established from the pressure drop/flow relationship, which to a first approximation may be simply the determined pressure drop divided by the flow immediately before the introduction of the occlusion.
A method is disclosed in U.S. Pat. No. 5,876,352 based on the recognition that a pressure drop is dependent effectively on a patient""s lung resistance and the compliance (resistancexe2x88x921) of the breathing circuit should be compensated for improving the accuracy of the described method.
Since the determined pressure drop effectively results from the patient tube resistance, then the known Blasius formula can then be used to provide, for a given gas flow F, a link between the determined pressure drop xcex94P, and the resistance R of the breathing tube of length L, and diameter D, according to:
xcex94P=0.24xc3x97(L/D4.75)xc3x97xcexc0.25xc3x97xcfx810.75xc3x97F1.75xe2x80x83xe2x80x83(1)
wherein xcexc and xcfx81 are respectively the gas viscosity and the gas density.
Equation (1) may be re-written as:
xcex94P=Kxc3x97(L/D4.75)xc3x97F1.75xe2x80x83xe2x80x83(2)
which gives
xcex94P/F75=Kxc3x97(L/D4.75)=Rxe2x80x83xe2x80x83(3)
Thus by determining a pressure drop xcex94P obtained at a flow value F, an indication of the effective resistance of the breathing tube can be obtained automatically during breathing assistance to avoid the need for user input of the information. In particular, from equation (3) a calculation of xcex94P/F1.75 at a known flow or a calculation of a value of a linear rate of change of xcex94P with F1.75, that is, dxcex94P/dF1.75, may preferably be used to provide an indication of the tube resistance.
Moreover the resistance is an actual resistance which thus reflects the reality of the breathing circuit in use. This has the advantage that the method may be employed to monitor the breathing circuit during mechanical breathing assistance for changes in resistance which would indicate a leakage (decreased resistance) or a blockage (increased resistance) or may be employed to provide a resistance value of the actual breathing circuit which is used to compensate pressure measurements made by sensors in the breathing circuit.
Preferably, the occlusions are introduced during an expiration phase of a patient breathing cycle so as to reduce the effect of the evaluation method on breathing gas supply to the patient and thus to reduce any discomfort which the patient might otherwise experience.
If the method is employed for a number of different gas flows, for example when calculating the flow dependent rate of change of pressure drop, that is either dxcex94P/dF1.75 or dxcex94P/dF, then the different gas flows may be obtained by introducing the occlusions at different times within one or more expiration phases of a patient breathing cycle. Since the flow during an expiration phase varies with time, this has the advantage that the natural variation of flow with time over an expiration phase may be utilized to further reduce the adverse effects that the inventive method may have on the provision of mechanical breathing assistance.
The evaluation may be made at different times within a single expiration phase so that a calculation of the flow dependent rate of change of the pressure drop may be made in a single brething cycle. Alternatively, if a calculation is made during an inspiration phase of a single breathing cycle, then a ramped gas flow can be provided and occlusions similarly introduced throughout that phase.
The above-stated object also is achieved in accordance with the present invention in an arrangement for carrying out the above-described method.